There are two general types of control systems for conventional ventilators. A first type delivers gas to a patient based on a frequency selected by the clinician that is independent of patient activity. This control system is used when the patient is non-alert, sedated, unresponsive or paralyzed. In this type of system, the ventilator is breathing for the patient. A second type of control system delivers gas to the patient in response to an inspiratory effort created by the patient. This type of ventilation helps the patient breathe. There are also ventilators and modes of ventilation that combine these two types of control systems. The present invention relates to ventilation systems and modes that respond to an inspiratory effort by the patient.
Control systems that respond to patient breathing efforts require breath sensors to detect inspiration. Conventional systems use pressure or flow sensors to detect the start of an inspiratory effort by the patient. The sensor is located somewhere in-line with the ventilation gas delivery circuit, either inside the ventilator, or in the tubing between the ventilator and the patient, or at the patient end of the tubing. In-line breath sensors are also used to measure the entire respiratory curve in addition to just the start of inspiration; however, because the gas being delivered by the ventilator also moves past the sensor, the sensor during that time no longer measures the patient's respiration but rather the ventilator activity. In a closed ventilation system, the patient lung pressure and the gas delivery circuit pressure, while not necessarily identical, are typically very close. In an open ventilation system in which the patient is also spontaneously breathing, the patient lung pressure and the gas delivery circuit pressure can be very different. In this case, a breath sensor in-line with the ventilation gas delivery circuit can be ineffective in measuring the entire respiratory pattern.
In ventilation systems in which the patient is expected to be breathing or partially breathing spontaneously, synchronization between the ventilator and the patient is important for comfort and efficacy. However, poor synchrony is still reported in some cases because of the demanding and exacting task of measuring all the different possible spontaneous breathing signals and the vast range of variations that exist.
Some attempts have been made to use sensors that are in parallel with the ventilation gas delivery system and are more directly coupled to the patient's actual respiration. The intent of these systems is to improve breath detection, to improve responsiveness of the ventilator, to improve the synchrony of the ventilator to the patient, or to reduce work of breathing required for a patient to trigger the ventilator.
For example, chest impedance sensors can be used to measure the entire respiratory curve of a patient and to use that signal to control the ventilator and synchronize the ventilator to the patient's breathing. However, this approach is technically challenging because the signal is prone to drift, noise and artifacts caused by patient motion and abdominal movement. In another technology, the neural respiratory drive measured with an esophageal catheter is used to measure the respiration of a patient. However, this technique requires an additional invasive device and sensor, and does not monitor exhalation activity since that is a neurally passive function.
Thermal intra-airway breath sensing is promising because it directly measures airflow in the trachea and, if implemented correctly, can determine the complete breathing pattern of the patient and can generate a breathing signal that is not disrupted by the ventilator gas flow.
Pressure-based breath sensing in the airway measures pressure at the distal end of an endotracheal tube, and using that pressure measurement to control a ventilator function for the purpose of reducing the patient's breath effort required for the patient to trigger a mechanical breath from the ventilator. Reduction in effort is a result of a quicker response time of the pressure signal because of the proximity of the signal to the patient's lung. While an improvement over conventional triggering techniques in conventional ventilation, this technique still has disadvantages and in fact has not yet been converted into commercial practice. For example, a sensor must have the necessary sensitivity and accuracy to detect light breathing pressures, while also withstanding high pressures so that it does not fail during a high pressure condition, such as a cough. This is especially of concern in medium and higher pressure ventilation delivery systems. Further, additional information related to the respiration pattern is desirable to increase the efficacy of the therapy. Also, existing systems have a logistically cumbersome interface with the external control system. In summary, existing systems have the one or more of the following disadvantages that require improvement: (1) they do not measure the complete breath cycle, (2) the are in-line with the channel used for ventilation gas delivery, (3) they have a limited range of accuracy and sensitivity, and (4) they are logistically cumbersome to interface with the ventilator.